As shown in FIG. 1, a typical computer system 10 has, among other components, a microprocessor 12, one or more forms of memory 14, integrated circuits 16 having specific functionalities, and peripheral computer resources (not shown), e.g., monitor, keyboard, software programs, etc. These components communicate with one another via communication paths 19, e.g., wires, buses, etc., to accomplish the various tasks of the computer system 10.
In order to properly accomplish such tasks, the computer system 10 relies on the basis of time to coordinate its various operations. To that end, a crystal oscillator 18 generates a system clock signal (referred to and known in the art as xe2x80x9creference clockxe2x80x9d and shown in FIG. 1 as sys_clk) to various parts of the computer system 10. Modem microprocessors and other integrated circuits, however, are typically capable of operating at frequencies significantly higher than the system clock, and thus, it becomes important to ensure that operations involving the microprocessor 12 and the other components of the computer system 10 use a proper and accurate reference of time.
Accordingly, as the frequencies of modem computers continue to increase, the need to rapidly transmit data between circuit interfaces also increases. To accurately receive data, a clock signal is often transmitted to help recover data transmitted to a receiving circuit by some transmitting circuit. The clock signal determines when the data should be sampled by the receiving circuit. Typically, the receiving circuit operates better when the clock signal is detected during the middle of the time the data is valid. To this end, a delay locked loop (xe2x80x9cDLLxe2x80x9d) is commonly used to generate a copy of the clock signal at a fixed phase shift with respect to the original clock signal.
FIG. 2 shows a portion of a typical computer system in which a DLL 30 is used. In FIG. 2, data 32 is transmitted from a transmitting circuit 34 to a receiving circuit 36. To aid in the recovery of the data 32 by the receiving circuit 36, a clock signal 38 is transmitted along with the data 32. To ensure that the data 32 is properly latched by the receiving circuit 36, the DLL 30 (which in FIG. 2 is shown as being part of the receiving circuit 36) regenerates the clock signal 38 to a valid voltage level and creates a phase shifted version of the clock signal 38. Accordingly, the use of the DLL 30 in this fashion ensures (1) that the data 32 is properly latched by triggering the receiving circuit 36 at a point in time in which the data 32 is valid.
FIG. 3 shows a configuration of a typical DLL 40. The DLL 40 uses a voltage-controlled delay line 42, composed of several delay elements 43, to delay an output clock signal, clk_out 45, with a fixed phase shift relative to an input clock signal, clk_in 44. A delay of the voltage-controlled delay line 42 is controlled by a feedback system including a phase detector 46, a charge pump 47, and a bias generator 48. The phase detector 46 detects any phase offset (i.e., phase difference) between the input clock signal 44 and the output clock signal 45 and then accordingly generates pulses on UP 49 and DOWN 51 signals that control the charge pump 47. Depending on the UP 49 and DOWN 51 pulses, the charge pump 47 transfers charge to or from a loop filter capacitor 53 via a control voltage signal, Vctrl 55. The bias generator 48 receives the control voltage signal 55 and produces bias voltages Vbn 57 and Vbp 59 that adjust the delay of the delay elements 43 in the voltage-controlled delay line 42. The DLL 40 is arranged such that the delay of the voltage-controlled delay line 42 attempts to maintain 180 degree phase shift between the input clock signal 44 and the output clock signal 45. For a more detailed background on the operation and behavior of a DLL, see J. Maneatis, xe2x80x9cLow-Jitter Process-Independent DLL and PLL Based on Self-Biased Techniques,xe2x80x9d IEEE Journal of Solid-State Circuits, vol. 31, no. 11, November 1996.
According to one aspect of the present invention, an integrated circuit comprises a phase detector arranged to detect a phase difference between a first clock signal and a second clock signal; a charge pump arranged to output a control voltage signal dependent on the phase difference; a capacitor operatively connected to the control voltage signal; a diode operatively connected to the capacitor; and a voltage-controlled delay line arranged to receive the first clock signal and output the second clock signal dependent on the control voltage signal.
According to another aspect, an integrated circuit comprises means for detecting a phase difference between a first clock signal and a second clock signal, means for generating a signal dependent on the phase difference, means for storing charge to maintain a voltage potential on the signal, a diode arranged to control a leakage current of the means for storing charge, and means for delaying the first clock signal in order to generate the second clock signal, where the means for delaying is dependent on the signal.
According to another aspect, a method for performing a delay locked loop operation comprises comparing a phase difference between a first clock signal and a second clock signal, generating a control voltage signal dependent on the comparing, storing charge dependent on the control voltage signal using a capacitor, controlling a leakage current of the capacitor with a diode positioned in series with the capacitor, and delaying the first clock signal to generate the second clock signal dependent on the control voltage signal.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.